This invention relates generally to geophysical prospecting and exploration, and more particularly relates to methods and means for evaluating seismic sequence lithology and property, and for using such evaluation to predict potential hydrocarbon reservoir, seal, trap or source, and the risk associated with such predictions.
It is well known that economic location and development of hydrocarbons is constrained by unprognosticated dry holes and high costs associated with such waste. Recent data compiled from existing exploration databases, e.g., compiled by Bec of Bruniquel, France and by S. N. Elf Aquitaine P., Pau, Dept. M.S.S., suggests that some 75% of exploration wells and over 50% of all wells drilled are dry. Of such dry wells, approximately 25% are associated with errors in spatial geometric prognosis of material evidence defined in seismic time and approximately 75% are caused by present inability of artisans to effectively apply geoscience to determine, within seismic sequences, those sedimentary rock properties identified in FIG. 2.
A finite number of single geological or geophysical causes exist with potential to modify the panoply of properties defined in FIG. 2 to the extent necessary to cause a substantially erroneous prognosis to occur. These causes or factors may be conveniently termed "geofactors" and all such geofactors which are present within a particular prospective basin and the like are relevant to the industrial risk equation. That is, there is inherently a risk involved with the prognostication, by those skilled in the art, where to engage in exploration for subsurface hydrocarbons.
Some geofactors are known or "visible" to artisans applying current technology and applied science, and may be quantified. Some geofactors, on the other hand, are "invisible," and beyond resolution using present methods and means known in the art. These invisible geofactors, therefore, compound the exploration problem (probably at an exponential or even a factorial rate), act and interact unpredictably, and are responsible for many, if not most, dry holes.
As is clear to practitioners in the art, between wells, and, therefore, in the subsurface areas where drilling investment is proposed, sequence property information in the form of physically-sampled evidence must be generated using a combination of interpolated available well data and data derived principally from seismic exploration. Well data allows measurement of many of the property parameters defined in FIG. 2.
To those familiar with the art, of course, seismic data allows good visibility of time information and acoustic impedance wave form data including amplitude, frequency and phase; it also provides fair information concerning velocity and density. But, unfortunately, seismic techniques historically have not provided an accurate route to conversion to spatial definition of sequence properties including lithology, porosity, etc. The seismic method presently allows the industrial working of material within a domain including location, time, velocity, depth, and acoustic impedence, in turn, including the wave form parameters, phase, frequency and amplitude. Historically, one "seismopetrophysically" defined set of properties, i.e., rock petrophysical property analysis using the seismic method, could equate to many lithologies such that porosity and other properties defined in FIG. 2 could not be accurately determined in areas of geologic complexity.
By the early 1980's, "normalization" procedures were in common industrial use to estimate normal burial change to seismic sequence sediment layer rock properties, as viewed through velocity behavior. See, for example, the following literature: Bulat et al., "Uplift determination from interval velocity studies, UK Southern N. Sea," Petroleum Geology of NW Europe, pp. 293-305, 1987; Wyllie et al., "Elastic Wave Velocities in Heterogeneous & Porous Media," Geophysics, vol. 21, #1, Jan. 1956, pp. 41-70; Gardner et al., "Formation Velocity & Density--The diagnostic basics for Stratigraphic Traps," Geophysics, vol. 39, #6, Dec. 1974; Brown, G., "Interval Velocity Studies in the Southern North Sea," GECO Exploration Services article; Feder, A. F., "Integrated interpretation for exploration," Oil & Gas Journal, May 5, 1986, pp. 180-187; Marsden, D., "I. Layer Cake Depth Conversion," Geophysics: The Leading Edge of Exploration, Jan. 1989, pp. 10-14; Ade et al., "F-Test, Isochron & Seismic Facies Analysis for Isopach Summation Depth Conversion," Singapore Seismic Stratigraphy Section, May 13, 1983; Carter, M. D., "II. Depth Conversion Using Normalized Interval Velocities," Geophysics; The Leading Edge, Jan. 1989, pp. 15-16.
By the mid 1980's, examination of prognosticated and actual drilling data revealed that abnormal burial history was the rule, not the exception, in prospective regions of many basins. Tectonic behavior variably in space and time, acts unpredictably to change in burial many physical properties including velocity, and also acts to disguise depositional properties. Tectonics acting to at least in part cause many traps also acts to disguise them. An inhomogeneous local pattern of stress and strain is associated with a presence of geofactors which may both influence deposition of rocks and influence the burial of rocks already deposited. These often complex influences on rock spatial occurrence and internal property act as an effective disguise to artisans and geoscientists working with prior art petroleum science and technology. As has been commonly reported in the international press in recent months, international oil and gas exploration is now conducted by private and state corporations who tend strongly to share similar technology.
All geofactors which are individually capable of causing geology as defined using seismic data, tend to be viewed as an "apparent geology". Apparent geology is that version of geology which is derived using the methods and means available within the evaluation process, and components of it are used in prognosis of prospects and traps. It may locally be different from real geology. The difference is commonly sufficient, over lateral distances per sequence of a k.m. or so, to significantly influence prospectivity, and apparent geology may subdivided into one of 3 groups:
[i] data resolution error, where perceived change is unreal, or real change is unperceived; PA1 [ii] real depositional behavior; and PA1 [iii] real burial changes to sequences. PA1 1. Time and "Apparent" Velocity PA1 2. Normalized Depth and SPIRAL Interpretation: Resolution; "Apparent Lithology"; Burial Deposition PA1 3. SPIRAL Lithology: Definition of reservoir volumes, etc. PA1 4. SPIRAL Reserve & Risk Determination
In the prior art, various rock properties critical to exploration and production, i.e., time geometric data and the parameters listed in FIG. 1, were prognosticated spatially with an accuracy related to the correctness of that prognosis as proven in drilling. There are some indications that as much as 56% of current wells are dry through errors in prognosis or risk of such properties. This is demonstrated in FIG. 3.
Improved interpretation of investment risk factors by those skilled in the art is precursed upon separate definition and analysis of these risk factors. The responsibility for analysis of such risk factors must be accepted by those who prepare and endorse evidence related to documentary requests for upstream expenditure. When a sufficient proportion of all relevant geological and geophysical factors can be adequately and separately defined, not only will risk be better understood and ultimately reduced, but also effective automation and computerization of the underlying complexities will be facilitated.
There has been a paucity of innovation in the prior art to enable a sufficiently comprehensive analysis of these geofactors to significantly reduce risk as herein described. But there has been a significant step taken by the instant inventor to better understand the velocities of acoustic energy in sedimentary rocks and for determining the causal effects thereof attributable to lithology and post depositional processes. More particularly, the methodology taught in U.S. Pat. No. 5,136,551 by the instant inventor and referred to hereinafter as "BECVEM" describes methods and means to improve analysis and definition of separate factors associated with the occurrence of dry holes, the disclosure of which is hereby incorporated herein by reference. BECVEM was developed to allow improved "normalization" to define and remove both normal burial effects common in a locally extensional environment, and anomalous effects caused by inversion, nonvertical compression, wrench tectonics, etc. Indeed, BECVEM has been tested by companies including Phillips Petroleum, Atlantic Richfield, Elf Aquitaine, Simon Petroleum, and PEP Ltd., of the University of Plymouth, U.K.
BECVEM's teachings allowed the testing of new, industrially useful methods and means to improve resolution from the seismic method, allowing interpretation and risk to incorporate spatial evidence of the action of each of several relevant geofactors indicated by seismic data collected at sites located between well control and in the vicinity of traps caused by localized tectonism.
BECVEM specifically augmented information locally relevant to hereinbefore defined apparent geology group [iii]. (Study sponsored by U.K. Dept. of Trade and Industry, O.S.O.; documented in Plymouth University Feasibility Study, June '93.) Thus, using BECVEM as a springboard, the present invention contributes to the prior art by enabling change in perception of apparent geology by changing perception of all three of these apparent geology groups, viz., [i], [ii], and [iii], and, insodoing, via performance of new industrial processes, engenders a new industrial "work domain" as will be hereinafter described in detail. (See FIG. 20)
To get better visibility of the subsurface requires not only that less ineffective work is performed, but also that more useful work is authorized. Accordingly, to constitute a significant improvement in the prior art requires a commensurate improvement of visibility, which, in turn, should reasonably be associated with a significant increase in useful industrial work load. As should be apparent to those skilled in the art, this requires innovative technology, since if effective methods and means already existed and were historically perceived as useful, such methods and means would now be industrially known and in use.
Because of the complexity of the causality for the panoply of geophysical properties defining the subsurface in a particular region or basin, those skilled in the art effectively utilizing the concepts embodied in BECVEM and disclosed herein typically will require ample training and the like. Thus, to effectively transfer such innovative and complex technology to industry requires computerization involving not only iterative interpretation of data collected via seismic testing and the like, but also requires integration with geophysical databases to properly validate and enhance knowledge for predicting suitable locations for hydrocarbon exploration and development. For such a contribution to the prior art to be an effective tool, it should provide a route to improved accuracy, allowing performance of the extra work necessary without extra costs and via use of existing industrial resource bases, overhead, staff, space, relevant material, and use of or integration with available industrial computer systems.
Such an advantageous route to translate within industrially acceptable levels of accuracy seismic geophysical parameters into geologically quality-controllable sequence petrophysics, i.e., physical classification of rock, has been heretofore unknown in the art. Indeed, as corroborated by such knowledgeable organizations as: Plymouth University, Devon U.K.; Marathon Exploration & Production U.K.; the U.K. Dept. of Trade and Industry (O.S.O.); the U.K. Petroleum Science & Technology Institute; Schlumberger Geoquest Systems, Inc.; Landmark Graphics Corp.; and Digicon Geophysical Corp.; no equivalent or comparable tools are either currently available or under research or development by those skilled in the art.
Accordingly, these limitations and disadvantages of the prior art are overcome with the present invention, and improved means and techniques are provided which are especially useful for reducing risks associated with geophysical prospecting and exploration by systematically and comprehensively evaluating and analyzing seismic sequence lithology and property under a novel work domain.